The present invention relates to particulate filters for diesel engines, of the type comprising a metal shell or casing which defines a cavity with an intake pipe and an outlet pipe, and a filtering body made of ceramic foam which is set in the path of communication between said intake pipe and outlet pipe.
A device of the type specified above is described and illustrated in the international patent application WO 00/69542 in the name of the present applicant, published on Nov. 23, 2000. The aforesaid prior patent application refers to a process for obtaining a filtering body for filtering particulate of diesel engines, which is made of a ceramic foam produced starting from a suspension (slurry) with a base of ceramic material, preparing a preform of expanded polymeric material and impregnating the said preform with the ceramic-powder slurry in such a way as to distribute the slurry evenly on the preform, and then subjecting the body thus obtained to a sintering heat cycle, so as to pyrolyse the polymeric material and obtain a high-porosity ceramic material. The powders of ceramic material are chosen from among mullite powder, alumina powder, zirconia powder, and mixtures thereof, which have a very high degree of purity, controlled grain size, and high specific surface.
In the embodiment of the particulate filter illustrated in the above-mentioned international patent, a metal shell or casing is provided, inside which there is set a tubular shell consisting of a ceramic pad, inside which is set a series of disks made of ceramic foam, which are obtained according to the process described above and which are set perpendicular to the direction of the flow of the exhaust gases through the tubular shell.
A drawback that has been encountered in the known filter mentioned above lies in the fact that it is not possible to obtain a large filtering surface without considerably increasing the overall dimensions of the device.
With a view to overcoming the aforesaid drawback, it would be desirable to make a filtering body having a tubular conformation, set inside the shell of the filter in such a way as to be radially traversed by the flow of the exhaust gases. A geometry of this sort would enable a considerable increase in the filtering surface even with relatively small increases in the axial length of the filtering body. However, tests carried out by the present applicant have shown that ceramic foams of the type in question cannot easily be formed in a tubular body with a relatively thick wall. Even if this problem were solved, the products thus obtained would have mechanical properties (above all in terms of resistance to vibrations) not adequate for their use as particulate filters.
The purpose of the present invention is to overcome the above problem providing a filter in which it is possible to use ceramic foams of the type forming the subject of the prior patent application in the name of the present applicant, and which, at the same time will guarantee the possibility of obtaining a large filtering surface with relatively small overall dimensions of the device.
With this purpose in view, the subject of the present invention is a particulate filter for diesel engines of the type specified at the beginning of the present description, characterized in that the aforesaid filtering body is made up of a plurality of separate elements made of ceramic foam and having a substantially plane shape, which are set about a longitudinal axis of the shell in such a way as to define, inside the shell, an inner chamber set within the array of filtering elements and at least one outer chamber set outside the array, the said chambers respectively communicating with the intake pipe and with the outlet pipe, or vice versa, in such a way that, during use, the flow of the engine exhaust gases that traverses the shell passing from the intake pipe to the outlet pipe is forced to traverse the aforesaid filtering elements, thus assuming a component of radial velocity with respect to the longitudinal axis of the shell. The flow may be either from the outside towards the inside (to maximize the intake filtering surface) or from the inside towards the outside.
Thanks to the aforesaid characteristic, it is possible, on the one hand, to obtain an arrangement that simulates the arrangement of a tubular filtering body, and, on the other hand, to form each of the filtering elements as a plane element, having a relatively thick wall, without any technological difficulty in the execution of the process.
Of course, even though the filter according to the invention is suitable for being used in a particularly advantageous way employing ceramic foams of the type forming the subject of the aforementioned international patent application, it is evident that equally falling within the scope of the invention is a filter having the above-described configuration but using filtering elements made of ceramic foams of a different type.
Thanks to the above-specified characteristics, the device according to the invention is able to present a large filtering surface, albeit having small overall dimensions.
In a preferred embodiment, the filter according to the invention is provided with two concentric arrays of filtering elements. In said preferred embodiment, the elements of each array are four in number and are set orthogonally in pairs facing one another, according to the sides of a rectangle. Preferably, the filtering elements of two adjacent sides rest along their adjacent edges on radial supports that project inwards from a cylindrical wall of the shell. Again preferably, a ceramic pad is set between each radial support and the elements resting thereon.
Furthermore, in the preferred embodiment, the aforesaid cylindrical wall is connected at one end to a bell-like structure having a neck that defines the aforesaid intake pipe (or outlet pipe), the said structure being closed on the opposite side by a diaphragm which has peripheral openings that force the flow of gas entering (or exiting) the shell to pass into the outer chamber set outside the filtering elements. The opposite end of the cylindrical wall is closed by a diaphragm which has a central neck that defines an outlet (or intake) pipe and communicates with the chamber set inside the array of filtering elements. Consequently, during use, the flow of exhaust gases that enters the device is forced to pass into an outer chamber set outside the two concentric arrays of filtering elements. From the said outer chamber the flow of exhaust gases is forced to pass into the inner chamber and to come out through the central opening set at the opposite end of the device, thus radially traversing the two arrays of filtering elements. The flow can be directed also in the opposite direction, i.e., from inside outwards.